1. Field of the Invention
The present invention relates to an image pickup apparatus, and a defect compensation device and a defect-compensating method, which are used in the image pickup apparatus.
2. Description of Related Art
In image pickup elements such as a charge coupled devices (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor, a plurality of pixels each for performing photoelectric conversion is provided. An image signal is generated based on a pixel signal that is generated by each of the pixels. In such the image pickup elements, there may be a defective pixel (hereinafter, referred to as “first defective pixel”) in which the pixel signal generated for each pixel results in a greater amount of offset than a level that has been previously set because of lack of uniformity on the element's characteristic. Further, there may be another defective pixel (hereinafter, referred to as “second defective pixel”) in which the pixel signal generated for each pixel results in a signal having a level that is independent on an incident beam because of a lattice defect thereof and the like. Accordingly, in an image pickup apparatus equipped with such the image pickup element, in order to prevent any picture quality from being made poor by these defective pixels, any defect compensation to compensate for a pixel signal corresponding to each of these defective pixels is performed in the image signal so that these defective pixels can be very little visible in an image.
In a case of the defect compensation of the pixel signal relative to the first defective pixel, an amount of the offset of the pixel signal relative to the first defective pixel is deleted from the pixel signal by using the compensation signal corresponding to the offset of the pixel signal relative to the first defective pixel. For example, if the pixel signal relative to the first defective pixel results in an offset OS of the pixel signal A (illustrated by oblique lines in FIG. 1) generated by the image pickup element, the offset OS can be deleted from the pixel signal A by subtracting a compensation signal HC having a same level as an amount of offset FL1 of the offset OS from the pixel signal A so that a pixel signal B shown in FIG. 1 can be generated. If, however, the offset OS of the pixel signal relative to the first defective pixel increases from the amount of offset FL1 to an amount of offset FL2 as a pixel signal C shown in FIG. 1 based on any fluctuation in temperature characteristic of the image pickup element, an offset OSR remains in a pixel signal D shown in FIG. 1 even when subtracting the compensation signal HC having the same level as an amount of offset FL1 of the offset OS from the pixel signal C. It is to be noted that signal levels LB shown in FIG. 1 and FIG. 2, which will be described in later, indicate a black level.
Japanese Patent No. 3104098 has disclosed an automatic defect compensation circuit of the image pickup element, in which a signal level of the compensation signal is changed based on the temperature changes of the image pickup element to compensate for the signal level of the image signal by using such the changed compensation signal, and when any defect is detected in the compensated image signal, the defect is concealed by a predetermined pre-sample image signal. If the pixel signal relative to the defective pixel has an increased level more than a predetermined level, any compensation of signal level using the compensation signal stops. Japanese Patent No. 3104098 and Japanese Patent Application Publication No. H06-113211 have disclosed that a memory stores any information relative to defective pixels and the defective pixels are compensated by using this information.